Appendix IL pressure, whichever be greater, but not both. As, however, an excess of fluid pressure would cause the joint to leak, the initial stress cannot be exceeded. Secondly, suppose an elastic packing be placed between the surfaces, and let us examine the problem by the elementary apparatus at c, Fig. 807. Between two walls d and e a light ^block / is supported, by a thin wire spring b and a strong india- rubber bar a] and let the walls be' separated so as to produce a tensile stress, say, of 10 Ibs. in each spring. Further, imagine that the wire spring is thus caused to extend J", while the rubber bar stretches J". The stress-strain diagram is shewn at D, where yy&=*iolbs., hj~Yi and/<£"=i"- Now let a force of 5 Ibs. be put upon the block f so as to pull it leftwards, thus increasing the stress in a and decreasing it in b. Shewing this on the diagram, produce gk and/,6: make £/=5lbs., draw Im || k h^ and mn\\ Ij. The shaded portion shews the new diagrams^ indicating the stress in b as 9 Ibs., and that in a as 14 Ibs. If now the force of 5 Ibs. be gradually increased, the stress in b will decrease; and when a force of 50 Ibs. is reached as at hp^ the spring is entirely freed from stress. The three cases are shewn by the static diagrams E, F, and G.